WASHINGTON (AP) -- The Pentagon is counting down to a rocket launch Friday night that could change the course of U.S. defense policy and present the next president with one of the most contentious international arms control debates in decades.

Although much is riding on the outcome the test of a national missile defense, Defense Secretary William Cohen is trying to tamp down expectations by saying it is not a make-or-break event.

In a sense, some missile defense critics agree. Regardless of the outcome, they believe President Clinton will move forward with building a nationwide shield against missiles.

Alaskans will be watching the test with keen interest. The Pentagon has indicated that an Alaska site -- either Fort Greely near Delta Junction or Clear Air Station near Nenana -- is the likely home base for the missile defense system if it is built.

In addition, a high-tech radar system would be built in the western Aleutians as an early-warning outpost.

Cohen stressed that Friday's test, the last before Clinton makes his decision, is only one in a series of more than a dozen that will ultimately determine the feasibility of defending all 50 states against a limited attack of ballistic missiles.

''We are trying to take it step by step because it's very, very difficult technology we are trying,'' Cohen told reporters Thursday in Tampa, Fla., where he presided over a change of command ceremony at U.S. Central Command.

The goal of the missile defense system is to destroy a hostile warhead in space by ramming it head-on with an interceptor missile.

''We are trying to hit a bullet with a bullet,'' Cohen said.

Many critics believe the technology is not feasible and that the Pentagon's testing methods are fatally flawed. Other critics say that even if it worked the weapon would not be worth the international outcry against it -- most notably Russia's threat to unravel other arms control treaties.

''Recent statements by Defense Secretary William Cohen indicate that the Clinton administration is on a path toward approval regardless of allied skepticism,'' the British American Security Information Council said Thursday.

Meanwhile, anti-nuclear activists were hoping to halt the test by positioning a ship in an area of the Pacific where a rocket stage is expected to splash down. Greenpeace planned to station a vessel about 110 miles offshore from Vandenberg Air Force Base, said Steve Shallhorn, the group's campaign director.

The Air Force has asked pilots and mariners to avoid the area during the test or risk damage or injury but said the test could continue even with a ship in the zone.

Greenpeace also set up camp outside Vandenberg's main gate, about 180 miles northwest of Los Angeles. And a group of protesters not affiliated with Greenpeace threatened to delay the launch by breaking into the base

The scenario for Friday's test is similar to that of the last, unsuccessful test in January: a target missile -- a Minuteman intercontinental ballistic missile with a dummy warhead -- launches from Vandenberg Air Force Base, Calif. Twenty minutes later, the interceptor rocket takes off from Kwajalein Atoll in the remote Marshall Islands in the Pacific Ocean. If it works as designed, a ''kill vehicle'' will detach from the interceptor rocket and guide itself into the path of the dummy warhead, destroying it by force of impact 144 miles above the earth.

The test, scheduled for as early as 10 p.m. EDT, depending on weather, has drawn unusually close attention around the world because it could set the stage for Clinton to give the go-ahead for building a full-scale missile defense. The one being tested uses prototype interceptors and radars.

Clinton has said he will await Cohen's recommendation, several weeks after the test, based on the Pentagon's assessment of the technical feasibility and cost of the project. The Pentagon has said a system using 100 interceptor missiles would cost about $36 billion, but a more robust system with multiple intercept bases and additional missile detection capabilities would run tens of billions more.

An independent panel of retired military officers and weapons experts told the Pentagon in a report last month that it believes missile defense is technologically feasible, but that the Pentagon may not be able to have a reliable system in place by 2005, the target date. The date is significant because the CIA has said it believes North Korea could have a long-range missile capable of reaching U.S. soil within five years.

Vice President Al Gore, the presumptive Democratic presidential nominee, favors Clinton's go-slow approach to missile defense, whereas his Republican challenger, Texas Gov. George W. Bush, says he would push for the earliest possible deployment of an even broader missile defense system, even if it meant abandoning the 1972 Anti-Ballistic Missile treaty, which prohibits national missile defense.

In Tampa, Cohen said he plans to discuss the U.S. plan with China's leaders when he travels to Beijing next week. China and Russia are outspoken opponents of the project. Both argue that deploying a U.S. missile defense would trigger a renewed arms race and undermine global stability.

Vladimir Yakovlev, the head of Russia's Strategic Rocket Forces, was quoted by the Interfax news agency Friday as saying the tests ''are the first step toward global nuclear instability.''

A U.S. missile defense system, he said, would ''lead directly to nuclear anarchy.''

Cohen said Clinton will take China's views into account when he makes his decision this summer or fall.

''Ultimately I think any president has to look at this situation and say, 'Can I afford to let the American people go undefended?''' Cohen said. The United States now has no way of stopping a long-range missile in flight.

Throughout the Cold War, the United States assumed that the Soviet Union would not launch a missile attack because it knew the United States would retaliate.

Now, the concern is that a smaller state like communist North Korea might launch a small number of missiles at the United States. The administration fears that North Korea -- and possibly Iraq and Iran at some point -- would be less rational than Soviet leaders were in considering the likelihood of U.S. retaliation.